These studies are designed to further characterize the interaction between soluble immune complexes and the IgG receptor on mononuclear phagocytes. Experiments will be carried out to determine the effect of antibody affinity on soluble complex structure and function. Soluble complexes will be prepared from antibody pools of varying affinities. Biological properties of these complexes to be examined include complement fixation, in vivo clearance and in vitro adherence to macrophages. In addition, experiments will be carried out in an attempt to further characterize the chemical nature of the macrophage receptor for IgG. Macrophage plasma membranes will be separated into various glycopeptide and glycolipid fractions. These fractions will be examined for receptor activity by inhibition experiments. Attempts will be made to isolate membrane fragments possessing receptor activity using immunoadsorption techniques and to chemically characterize the IgG receptor.